Washover
Washover in petroleum drilling and workover operations is a fishing technique used to free stuck pipe (drill pipe, drill collars, casing, or tubing) that is mechanically held in the wellbore by cement, junk, formation collapse, or differential pressure sticking, by running a washover tool (also called an overshot or washover pipe) over the outside of the stuck tubular and milling or cutting through the cement, junk, or formation material holding the stuck pipe in place, allowing it to be freed and retrieved from the wellbore; the washover assembly consists of washover pipe (a larger-diameter pipe than the stuck tubular, able to telescope over the outside of the stuck pipe with an annular gap sufficient to accommodate the milling or cutting action), a washpipe shoe (the lower-end fitting that contains the cutting or milling elements that contact the material holding the stuck pipe), and connecting components to the surface drill string; the principle of washover operations is fundamentally different from straight overshot fishing (which grabs the top of the stuck fish and attempts to pull it out) in that washover addresses the external constraint on the stuck tubular rather than the tensile strength of the tubular itself, removing the material that is binding the pipe from the outside so that the pipe can be freed by pull or rotation after the washover has progressed to a depth below the stuck point; washover operations are conducted when the fish is cemented in place, packed off with collapsed formation, or held by debris in the annulus between the fish and the casing, situations where simple pulling and jarring on the fish fail to achieve sufficient tensile load to release the stuck tubular without risking parting the pipe above the stuck point.
Key Takeaways
- Washover shoe design and cutting structure selection for the washover tool determines the types of material the tool can effectively mill through to free the stuck pipe: the standard mill-type washover shoe uses tungsten carbide inserts or matrix-impregnated blades similar to a junk mill shoe, with the cutting action applied to the material in the annular space between the washover pipe ID and the stuck fish OD as the washover assembly is rotated with mild downward weight; a burn shoe (also called a rotary shoe or scraper shoe) uses internal blades that both scrape the outside of the fish to clean it for packer grip and mill through the surrounding debris; a milling shoe with full-face cutting coverage is used when the annular space is packed with cement or hard formation fragments that require aggressive cutting to advance the washover below the stuck point; the washover shoe OD must be sized to pass through the inside of the casing (with adequate clearance) while the washover pipe ID must be large enough to telescope over the fish OD with sufficient annular gap for the milling action and for circulation of the milling debris out of the annulus via the drilling fluid pumped down the washover pipe and up the annulus outside the washover pipe above the shoe; a common challenge is that the washover tool must clear both the casing drift ID at the top and the fish OD at the bottom simultaneously, constraining the washover pipe wall thickness to the difference between these two dimensions.
- Free point determination before initiating a washover operation identifies the depth at which the stuck pipe transitions from free (movable) to stuck (immovable), allowing the washover operation to target the stuck interval rather than washing over a section of pipe that is already free: the free point is determined by a combination of surface measurements (applying increasing tension and torque at surface while monitoring how much stretch and rotation are observed in the top of the string, and correlating the observed string compliance with the free pipe elastic stretch and torque transmission equations to identify the depth below which no compliance is observed) and wireline logging (the free point indicator log, which uses a strain gauge tool attached to the pipe to measure the local pipe extension and torque transmission under applied loads, identifying the exact depth at which the pipe transitions from free to stuck); knowing the free point allows the fishing engineer to design a washover operation that starts just above the free point (not at a shallower depth where the pipe is already free) and advances downward to a depth below which the pipe can be pulled free after the washover removes the external constraint; washing over a longer section than necessary adds time and risk to the operation without improving the probability of freeing the fish, and the goal is to advance the washover to the minimum depth that will allow a successful pull to free the pipe.
- Cement washover operations are conducted when drill pipe or casing has been cemented in place during an unplanned cementing event (cement squeezed into the annulus above a packer from a failed primary cement job, or cement that has risen higher in the annulus than planned and covered the drill pipe before it could be pulled) and the cement has set to the point where the pipe can no longer be rotated or pulled free: the washover assembly is run over the cemented pipe and the washpipe shoe mills through the hardened cement in the annulus between the fish and the casing, progressively advancing downward until the entire cemented interval has been washed over and the pipe is free from the cement that was binding it; cement washover is one of the most time-consuming and expensive fishing operations because cement develops high compressive strength (typically 3,000-8,000 psi after 24-48 hours of curing) that requires significant milling energy to remove, and the milling rate in hard cement may be only 1-3 feet per hour, making a 100-foot cemented interval a 30-100 hour washover operation at significant daily rig rate; the cement washover rate can be improved by using a chemical wash (pumping an acid solution or a cement-retarding chemical through the washover to dissolve or soften the cement before the mechanical milling action reaches it), but the effectiveness of chemical washing depends on the cement permeability and the volume of chemical that can be placed in contact with the cement surface.
- Differential pressure sticking response using washover addresses the specific situation where the drill pipe has been stuck against the filter cake on the borehole wall by the differential pressure between the wellbore fluid and the formation (the same differential pressure that drives filtrate invasion), with the pipe embedded in the soft filter cake and held by the net area of the pipe face against the permeable zone times the pressure differential: standard remediation for differential sticking involves spotting a soak (a low-friction oil or surfactant blend spotted in the annulus around the stuck zone) that reduces the pressure differential and lubricates the interface between the pipe and the cake, and pulling and jarring on the pipe while the soak has time to work; if these standard approaches fail after 24-48 hours and the stuck point is confirmed by free point analysis to be in the filter cake zone, a washover operation that cuts the pipe just above the stuck point (using a chemical cutter or mechanical string shot), allows the upper portion to be retrieved, and then washes over the remaining stuck portion can free the lower section by milling the filter cake and surrounding material from the outside of the stuck pipe; the decision to initiate a washover for differential sticking versus continuing to soak and jar depends on the economic balance between the rig time cost of extended soak operations and the additional fishing time and complexity of the washover operation.
- Back-off operations combined with washover allow the retrieval of the portion of the drill string above the stuck point by unscrewing the connection just above the stuck zone (back-off), pulling the free portion to surface, and then initiating the washover on the remaining stuck fish without the weight and length of the free portion above complicating the washover tool handling: the back-off is achieved by applying a reverse torque to the string while a string shot (a detonating cord charge run inside the pipe on wireline) is fired at the connection above the stuck point, with the combination of reverse torque and the percussion wave from the string shot causing the connection to unscrew cleanly at that specific connection; a successful back-off leaves a fish consisting only of the stuck pipe from the back-off connection downward, which is a much more manageable washover target than a fish that includes the entire stuck section plus tens of thousands of feet of free drill pipe above it; the back-off depth selection (choosing which connection to back off at, just above the stuck point) requires accurate free point analysis to ensure the back-off point is in the free pipe section, because attempting to back off at a connection that is itself stuck will fail and may damage the string further.
Fast Facts
Washover tools have been a part of the oilfield fishing toolkit since the early decades of rotary drilling, when the reliability of drill pipe connections and cementing operations was significantly lower than today and stuck pipe was a common occurrence requiring remediation. The development of tungsten carbide cutting structures for washover shoes in the mid-20th century significantly improved the milling rate through cemented intervals and hard debris, reducing the time required for washover operations from what had previously been measured in weeks to what can now typically be accomplished in days. The economic pressure to minimize fishing and washover time, since the rig rates for deepwater and HPHT wells can exceed $500,000 per day, has driven the development of improved drilling fluid programs (to reduce sticking risk), better fishing tool designs, and more rigorous stuck pipe prevention protocols that have reduced the frequency of washover operations in modern well construction even as the technical sophistication required when they do occur has increased.
What Is Washover?
Washover is the fishing technique that frees stuck pipe from the outside in, rather than from the top down. Where a conventional fishing overshot grabs the top of the stuck fish and pulls, washover runs a larger-diameter pipe over the outside of the stuck tubular and mills through whatever is holding it in place, whether cement, formation debris, or packed cuttings, progressively freeing the pipe from its external constraint until enough of the stuck interval has been washed over that a straight pull or jar can bring the fish to surface. It is the drilling equivalent of cutting the frame off a picture that is stuck to the wall rather than trying to pull the picture off by its edges. Washover operations are slow, expensive, and technically demanding because the washover tool must telescope over the stuck fish (which means it must be larger than the fish but smaller than the casing ID), mill effectively through hard or irregular material in the confined annular space, and maintain adequate circulation to remove the milled debris without packing the annulus. When a drill string is cemented in place or packed off by collapsed formation over a significant interval, washover may be the only technically viable path to recovering the fish and resuming drilling operations.